<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var num =10;
        // fun();
        // function fun(){
        //     console.log(num);
        //     var num = 20;
        //  } 
        //相当于代码如下
        var num;
        function fun(){
            var num;
            console.log(num);
            num = 20;
        }
        num=10;
        fun();

    </script>
</body>
</html>